Initial reorg.

This is largely the same as #305, but updated for the current tree.

crates/wasi-common/src/sys/mod.rs

@@ -0,0 +1,24 @@
+use crate::wasi;
+use cfg_if::cfg_if;
+
+cfg_if! {
+    if #[cfg(unix)] {
+        mod unix;
+        pub(crate) use self::unix::*;
+        pub use self::unix::preopen_dir;
+
+        pub(crate) fn errno_from_host(err: i32) -> wasi::__wasi_errno_t {
+            host_impl::errno_from_nix(nix::errno::from_i32(err)).as_wasi_errno()
+        }
+    } else if #[cfg(windows)] {
+        mod windows;
+        pub(crate) use self::windows::*;
+        pub use self::windows::preopen_dir;
+
+        pub(crate) fn errno_from_host(err: i32) -> wasi::__wasi_errno_t {
+            host_impl::errno_from_win(winx::winerror::WinError::from_u32(err as u32))
+        }
+    } else {
+        compile_error!("wasi-common doesn't compile for this platform yet");
+    }
+}